There is a wide ranging demand for increased communication capabilities, including more channels and greater bandwidth per channel. The demand stems from the popularity of the Internet as well as entertainment and information services that require multiple channels and greater bandwidth capabilities per channel. One preferred transmission medium for high bandwidth transmissions is optical fiber cables. Optical fiber cables arc widely used for long distance transmission such as between two cities as well as extremely short range applications including data communications between functional blocks in a semiconductor circuit. However, optical fiber cables are less commonly used within buildings or homes.
A typical single optical fiber includes an inner glass core coated with a cladding material. The cladding material has a lower index of refraction than the core to promote total internal reflection of light propagating down the core. The cladding, in turn, is typically covered with one or more protective coatings. Often, two protective coatings including a primary layer and a secondary layer are utilized. Both primary and secondary layers are preferably UV-curable polymeric coatings.
The primary layer, or the layer that engages the cladding, is typically softer and more flexible than the secondary or outer layer which is harder for purposes of protecting the optical fiber during use. The primary and secondary coatings may be combined into a single coating, which are often referred to as buffer coatings.
A typical single mode optical fiber has a 9 μm diameter glass core and an outer cladding having a diameter of about 125 μm. Multiple mode fibers have a larger diameter core, ranging from 50 to 100 μm while the outer cladding diameter for multiple mode fibers ranges from 125 to 140 μm. The coatings for such optical fibers are typically available in two sizes, 250 μm and 900 μm.
When optical fiber cables are employed in structures such as buildings or homes in the form of short length cables or in local area networks, the optical fibers are usually bundled in multiple fiber packages referred to as premises cables. When such premises cables are used within a structure, modem building codes require that the premises cables meet specific flame-retardant guidelines.
To satisfy the flame-retardant guidelines, a number of strategies have been employed. One strategy is to add an additional flame-retardant layer on top of the single buffer layer or the primary and secondary coating layers. Such a strategy may not satisfy the guidelines if the buffer, primary and/or secondary layers are not flame-retardant. Thus, manufacturers have sought a single coating applied directly to the cladding layer and that is flame-retardant. However, as noted above, in addition to being flame-retardant, such a single buffer coating would also have to be sufficiently flexible to avoid damage when the optical fiber cable is bent and sufficiently hard to protect the optical fiber cable during use. Further, to reduce manufacturing cost, the buffer layer should be UV-curable.
Current UV-curable and flame-retardant materials for buffer layers are acrylate-based. Acrylate-based materials have fast UV-curable properties. However, in order to make an acrylate coating or buffer layer flame-retardant, inorganic materials such as aluminum hydroxide or potassium hydroxide, in particulate form, are added to the acrylate formulation. Adding particulate materials to an uncured liquid precursor to the buffer layer creates specific problems. For example, the addition of particulate materials adversely affects the structural integrity of the cured film. Yet another problem associated with the addition of particulates to an uncured liquid is that the particulate matter can settle out of the formulation prior to distribution around an optical fiber and therefore the particulate matter can be unevenly distributed throughout the buffer layer resulting in inconsistent properties of the cured buffer layer. Furthermore, the flame-retardant properties of acrylate films containing aluminum hydroxide or potassium hydroxide are only marginal at the level that the aluminum or potassium hydroxide materials can be distributed into the buffer layer formulation. Still further, inorganic material such as aluminum or potassium hydroxide adsorb moisture readily thereby undesirably increasing the tackiness of the surface of the cured buffer coating.
Therefore, there is a need for an improved single or multiple layer buffer coating for optical fibers that provides the requisite flexibility, hardness and flame-retardant properties and that is also UV-curable.